Ampeg SVT 3 pro tube preamp section troubleshooting

[JeezyMcNuggles]

So did you get the exact same fan as stock replacement from Ampeg, or a different fan? I think they have it on digikey or something. Some engineering genius on another board tested a few and posted their noise levels.

As for the horse guy, I don't care if he's the head of NASA -- his posts are not helpful to the people asking -- his posts are self-serving to the point of parody, off-topic, and always wrong. I knew he would chime in here with "do you even OWN an oscilloscope!?" in total ignorance of this amp -- the other guy had the right answer about the tube gain, and another guy humbly posted detailed instructions for biasing the amp. BTW "righty tighty, lefty loosey" is exactly right - good one.

Nah, I can't find it. I'll pop the cover off my amp, and give it to you. I need to set my bias again anyway.

 

[Paul Szegedin]

Nah, I can't find it. I'll pop the cover off my amp, and give it to you. I need to set my bias again anyway.

Thanks for looking. I opened up, cleaned and lubed the fan - 3o minutes of my time on earth that I won't get back - helped a little but not much. I also called Ampeg - the guy was nice but not especially helpful. Just gave me the part number of the direct replacement. He said he knows there are much quieter fans than the ones they sourced, but he wouldn't name one. Told me it just has to be the right size, voltage, and in the ballpark of flow rate. I was like, 'can I get away with 10-15% less flow rate on a quieter fan? Blink twice for yes.'

Good luck with your re-biasing.

 

[agedhorse]

So did you get the exact same fan as stock replacement from Ampeg, or a different fan? I think they have it on digikey or something. Some engineering genius on another board tested a few and posted their noise levels.

As for the horse guy, I don't care if he's the head of NASA -- his posts are not helpful to the people asking -- his posts are self-serving to the point of parody, off-topic, and always wrong. I knew he would chime in here with "do you even OWN an oscilloscope!?" in total ignorance of this amp -- the other guy had the right answer about the tube gain, and another guy humbly posted detailed instructions for biasing the amp. BTW "righty tighty, lefty loosey" is exactly right - good one.

There are 2 common issues with this amp, a scope determines in 39 seconds where the problem isn’t. That’s why I asked if he owned a scope.

Twiddling with the bias control won’t fix the other problem and several members here have blown their amps up accidentally by making a “bias misstep”.

I’m not sure what your problem is with me, or why you have a problem with me providing accurate information to others in the community.

Do you really think it’s ok to provide bad and wrong information? There’s no need for somebody here to damage their amp (or get hurt) from bad advice.

 

[JeezyMcNuggles]

There are 2 common issues with this amp, a scope determines in 39 seconds where the problem isn’t. That’s why I asked if he owned a scope.

Twiddling with the bias control won’t fix the other problem and several members here have blown their amps up accidentally by making a “bias misstep”.

I’m not sure what your problem is with me, or why you have a problem with me providing accurate information to others in the community.

Do you really think it’s ok to provide bad and wrong information? There’s no need for somebody here to damage their amp (or get hurt) from bad advice.

You do rub people wrong sometimes. I do too but one thing is for absolute sure about you bro, you never steer anyone into destroying thier amplifiers, or getting hurt. I tell people to go for it hahaha. I did it. Do it up! Hahahaha

But, I'm also pretty resourceful. I can usually figure it out.

 

[JeezyMcNuggles]

Thanks for looking. I opened up, cleaned and lubed the fan - 3o minutes of my time on earth that I won't get back - helped a little but not much. I also called Ampeg - the guy was nice but not especially helpful. Just gave me the part number of the direct replacement. He said he knows there are much quieter fans than the ones they sourced, but he wouldn't name one. Told me it just has to be the right size, voltage, and in the ballpark of flow rate. I was like, 'can I get away with 10-15% less flow rate on a quieter fan? Blink twice for yes.'

Good luck with your re-biasing.

Here you go bro

Sprint DC
Model: ST24B3

 

[Engle]

Gotta love that transformer!

 

[JeezyMcNuggles]

There are 2 common issues with this amp, a scope determines in 39 seconds where the problem isn’t. That’s why I asked if he owned a scope.

Twiddling with the bias control won’t fix the other problem and several members here have blown their amps up accidentally by making a “bias misstep”.

I’m not sure what your problem is with me, or why you have a problem with me providing accurate information to others in the community.

Do you really think it’s ok to provide bad and wrong information? There’s no need for somebody here to damage their amp (or get hurt) from bad advice.

Question Andy, on your amps, is the fan supposed to draw air in, or expell the air that's drawn in from the sides? Cause I just noticed I have my fan pulling air in. I thought I had installed I the same way I pulled the other out, but now I'm second guessing myself. I replaced it quite a few months ago, with no overheating issues thus far.

 

[JeezyMcNuggles]

Thanks for looking. I opened up, cleaned and lubed the fan - 3o minutes of my time on earth that I won't get back - helped a little but not much. I also called Ampeg - the guy was nice but not especially helpful. Just gave me the part number of the direct replacement. He said he knows there are much quieter fans than the ones they sourced, but he wouldn't name one. Told me it just has to be the right size, voltage, and in the ballpark of flow rate. I was like, 'can I get away with 10-15% less flow rate on a quieter fan? Blink twice for yes.'

Good luck with your re-biasing.

Okay, I'll ask you. Is your fan pushing air out the back, or drawing air in from the back?

 

[Paul Szegedin]

Okay, I'll ask you. Is your fan pushing air out the back, or drawing air in from the back?

Hey thanks man. Mine was the ST24A3. Glad to know that a new one will be quieter.

Mine was pushing back, as in over the transformers/resistors, which is how it should be -- that or pushing straight down over them from the top also works . If your fan was installed backwards sucking air in, I guess that means that it doesn't need a heck of a lot of cooling at moderate volumes. I've yet to see mine speed up at all. Some SS amps don't have fans at all.

 

[JeezyMcNuggles]

Hey thanks man. Mine was the ST24A3. Glad to know that a new one will be quieter.

Mine was pushing back, as in over the transformers/resistors, which is how it should be -- that or pushing straight down over them from the top also works . If your fan was installed backwards sucking air in, I guess that means that it doesn't need a heck of a lot of cooling at moderate volumes. I've yet to see mine speed up at all. Some SS amps don't have fans at all.

Right on. Okay, so I did install it backwards. Ha! Well, havent had a problem with that yet

 

[JeezyMcNuggles]

Hey thanks man. Mine was the ST24A3. Glad to know that a new one will be quieter.

Mine was pushing back, as in over the transformers/resistors, which is how it should be -- that or pushing straight down over them from the top also works . If your fan was installed backwards sucking air in, I guess that means that it doesn't need a heck of a lot of cooling at moderate volumes. I've yet to see mine speed up at all. Some SS amps don't have fans at all.

Aight, bias back up. It had dropped down to 15ish. The distortion wasn't bad, I could notice it. But, now she'll be running like new. I'm gonna swap out the pot for a multi turn pot. Actually solve that stupid problem

 

[agedhorse]

Question Andy, on your amps, is the fan supposed to draw air in, or expell the air that's drawn in from the sides? Cause I just noticed I have my fan pulling air in. I thought I had installed I the same way I pulled the other out, but now I'm second guessing myself. I replaced it quite a few months ago, with no overheating issues thus far.

Question Andy, on your amps, is the fan supposed to draw air in, or expell the air that's drawn in from the sides? Cause I just noticed I have my fan pulling air in. I thought I had installed I the same way I pulled the other out, but now I'm second guessing myself. I replaced it quite a few months ago, with no overheating issues thus far.

It doesn’t really matter if the fan pushes or pulls, it’s all about airflow versus static pressure.

Getting into the technical weeds, the fan pushing towards the back creates positive pressure due to the restrictions of the perforated metal back, the punched tabs on the sides create a little bit of leakage and turbulence but the difference is usually within 10%.

 

[agedhorse]

Aight, bias back up. It had dropped down to 15ish. The distortion wasn't bad, I could notice it. But, now she'll be running like new. I'm gonna swap out the pot for a multi turn pot. Actually solve that stupid problem

That’s only a small part of the problem. The scaling of the Vbe multiplier is way too sensitive. There may have been a TSB or ECO on this.

I hope I haven’t offended anyone with the information I just provided.

 

[JeezyMcNuggles]

It doesn’t really matter if the fan pushes or pulls, it’s all about airflow versus static pressure.

Getting into the technical weeds, the fan pushing towards the back creates positive pressure due to the restrictions of the perforated metal back, the punched tabs on the sides create a little bit of leakage and turbulence but the difference is usually within 10%.

Thats good to know. I flipped it anyway, and got my current back up to speed. She's good as new til she's not again. It sucks too, cause this amp sounds so killer. And it has great features. They really need to fix this stupid bias drift problem. I'm gonna swap the pot out for a multi turn.

I did actually talk to Dino about that though. He's actually never heard of the problem. Maybe they'll start looking into it now...maybe

 

[Paul Szegedin]

Aight, bias back up. It had dropped down to 15ish. The distortion wasn't bad, I could notice it. But, now she'll be running like new. I'm gonna swap out the pot for a multi turn pot. Actually solve that stupid problem

Sweet. If you do that pot, would be great if you could post pics of the process so others know how it goes and the part source. I wonder how much hassle it is (whether it's worth it) to get to the under side of that board.

Pulled my 40 year old GK back out, and not sure this Ampeg is really any better, but enjoying the journey of tinkering and learning new things, because this is how you do it -- not in engineering school. Feels good to successfully DIY, not pay a tech, amirite?

 

[agedhorse]

Thats good to know. I flipped it anyway, and got my current back up to speed. She's good as new til she's not again. It sucks too, cause this amp sounds so killer. And it has great features. They really need to fix this stupid bias drift problem. I'm gonna swap the pot out for a multi turn.

I did actually talk to Dino about that though. He's actually never heard of the problem. Maybe they'll start looking into it now...maybe

The range is impacted even more by the steepness of the Vgs curve as the device begins to conduct.

 

[agedhorse]

but enjoying the journey of tinkering and learning new things, because this is how you do it -- not in engineering school. Feels good to successfully DIY, not pay a tech, amirite?

Sure you can tinker and maybe learn something, but to really understand how something works and why designs are done the way they are, education is very useful. Even if it involves studying online electronics courses and tutorials.

It may feel good to be successful, but how does it feel when it doesn’t work out and it would have been less expensive to have a (qualified) tech repair it properly?

Is there a particular issue you have about getting an education? Do you have something against the engineers?

 

[JeezyMcNuggles]

Sweet. If you do that pot, would be great if you could post pics of the process so others know how it goes and the part source. I wonder how much hassle it is (whether it's worth it) to get to the under side of that board.

Pulled my 40 year old GK back out, and not sure this Ampeg is really any better, but enjoying the journey of tinkering and learning new things, because this is how you do it -- not in engineering school. Feels good to successfully DIY, not pay a tech, amirite?

It's not hard. I've pulled that board out already. I swapped the di port, input jack, and mid control knob already. I got my amp as a beat up fixer myself.

 

[JeezyMcNuggles]

Sure you can tinker and maybe learn something, but to really understand how something works and why designs are done the way they are, education is very useful. Even if it involves studying online electronics courses and tutorials.

It may feel good to be successful, but how does it feel when it doesn’t work out and it would have been less expensive to have a (qualified) tech repair it properly?

Is there a particular issue you have about getting an education? Do you have something against the engineers?

There's people that learn from instruction, and people that learn for themselves. I, for one, learn for myself. Then, there's those who refuse to do anything for themselves

 

[JeezyMcNuggles]

Sweet. If you do that pot, would be great if you could post pics of the process so others know how it goes and the part source. I wonder how much hassle it is (whether it's worth it) to get to the under side of that board.

Pulled my 40 year old GK back out, and not sure this Ampeg is really any better, but enjoying the journey of tinkering and learning new things, because this is how you do it -- not in engineering school. Feels good to successfully DIY, not pay a tech, amirite?

I ordered a bourns 10kohm 25 turn pot. 2 bucks plus shipping from mouser. Looking around, this one seems to be the closest match to what's in there, except that you can turn the knob 25 times from - to +, instead of once. Should help. Even if it still drifts because of a design flaw, it should take longer, and be far more manageable.